Amplifier device with digitally controllable gain and optical disc read apparatus incorporating such a device

ABSTRACT

The device has reference current sources (T 71 , T 72 , T 7M ) which are arranged in such a way that the sum of the currents flowing through each of the said sources is equal to the input current (I IN , I′ IN ) and output current sources (T 81 , T 82 , T 8N ), in each of which the current flowing through the reference sources is duplicated and which are arranged in such a way that the sum of the currents flowing through the output sources is equal to the output current (I OUT , I′ OUT ). The number of reference sources (T 71 , T 72 , T 7M ) and output sources (T 81 , T 82 , T 8N ) which are connected is controlled by the digital signal ( 200 ) and determines the gain of the device. Application to the circuit for processing the signals output by a read head of an optical disc reader apparatus.

BACKGROUND OF THE INVENTION

1 Field of the Invention

The present invention relates to an amplifier with digitally controlledgain and its application to a circuit for processing signals output by aread head of an optical disc reader apparatus.

2 Description of the Related Art

Referring to FIG. 1, information recorded on an optical disc 1 is read,in a manner which is known per se, by projecting a light beam 3 outputby a laser diode 2 onto the reflective surface of the disc. Thereflected beam 4 is detected by photodiodes A, B, C, D, E and F, thesignals output by these photodiodes being used, inter alia, to decodethe information stored on the optical disc. The photodiodes A to F, aswell as the laser diode 2, form part of an optical and mechanicalassembly 5 referred to as the read head of the apparatus, or opticalpick-up, which does not form the subject-matter of the present inventionand will not be described in further detail.

The signals output by the photodiodes A and C are added before beingprocessed. The same is true of the signals output by the photodiodes Band D. Four signals S0 to S3 thus leave the read head 5 to be processedand decoded by a suitable processing circuit. In the case in point, thesignals S0 to S3 correspond to the currents flowing through thephotodiodes. These currents vary as a function of a certain number offactors, such as the reflectivity of the surface of the disc (dependingon its state of cleanliness) or the power of the emitting laser diode 2.

Fixed-gain current/current amplifiers 10 to 13 receive the signals S0 toS3 and are connected to the four inputs E0 to E3 of a multiplexer 15.The output of the multiplexer 15 is connected to a resistor 16, whichconverts the current into voltage, and to the input of analogue/digitalconverter 18 which delivers a digital signal encoded using six bits to adigital circuit 25 for processing and decoding the signals emitted bythe photodiodes.

In order to make maximum use of the dynamic range of theanalogue/digital converter 18, and thus to deliver high-quality signalsto the circuit 25, it is necessary for the signal entering the converterto have a sufficient amplitude. However, this amplitude depends directlyon the current which is picked up by the photodiodes A to F. Dependingon the type of read head which is used, the amplitude of the signalreceived by the analogue/digital converter 18 can vary from one to fourtimes. These large variations are corrected by modifying the value ofthe resistor 16 as a function of the type of read head used. To thatend, when the circuit for processing the signals output by the read head5 is produced in integrated circuit form, a pin is provided on thecircuit in order to make it possible to connect an external resistor 16of suitable value to the read head.

However, the value of the resistor 16 is fixed for each given type ofread head and does not make it possible to correct smaller but all thesame significant variations in the current output by the photodiodes,which variations are due to the surface condition of the optical disc 1,which is not uniform (fingerprints, etc.) or the conditions under whichthe apparatus is being used (temperature, background light, etc.).

To solve this problem, it has been proposed in the prior art to modifythe current flowing through the laser diode 2 as a function of thedigital signal which is obtained at the output of the analogue/digitalconverter 18, so as to correct a decrease in the current output by thephotodiodes by increasing the current in the laser diode, and viceversa. To do this, the digital signal output by the converter 18 is sentto the input of a digital/analogue converter 20, the analogue signal 8resulting from the conversion being used to control the current in thelaser diode 2.

However, this arrangement is unsatisfactory for the following reasons:on the one hand, it is necessary to add a digital/analogue converter 20,which increases the size of the assembly; on the other hand, thevariations in the current in the laser diode lead to a significantreduction in its life.

It is furthermore known to use, in analogue/digital processing systems,a gain adjustment stage in order to match the amplitude of the incominganalogue signal to the dynamic range of the analogue/digital converterswhich are used in the processing system. FIGS. 2a to 2 c illustratedifferent ways of adjusting the gain of an amplifier. In these figures,the amplifiers are voltage-controlled.

In FIG. 2a, an input signal V_(IN1) is applied to one of the gates of atwo-gate MOSFET transistor 36 whose drain is connected to a supplyV_(DD) via a resistor 34 and whose source is earthed. The drain of thetransistor 36 is also connected to an operational amplifier 37, set upin follower mode, whose output is connected to the input of ananalogue/digital converter 38 which delivers a digital signal to adigital processing circuit 35. In order to adjust the amplitude of thesignal entering the converter 38, the transconductance of the transistor36 will be modified by applying, to the second gate of the transistor, acontrol voltage V_(COM1) coming from a return loop. The return loop isformed by a digital signal leaving the circuit 35 sent to adigital/analogue converter 39 whose output is connected to anoperational amplifier 37′, set up in follower mode, which delivers at anoutput the control voltage V_(COM1). The problem with this device isthat the gain does not vary linearly as a function of the controlvoltage throughout the adjustment range of the gain.

In FIG. 2b, elements which are identical have the same references as inFIG. 2a. In this device, the input signal V_(IN2) is applied to a firstnon-inverting input of an operational amplifier 47 whose inverting inputis earthed via a resistor 44 and which has a feedback loop connectingits output to its inverting input via a MOS transistor 46 set up intriode mode. The gain of the amplifier is modified by a control voltageV_(COM2) (taken from the output of the operational amplifier 37′) whichmodifies the value of the resistor formed by the MOS transistor 46. Thisdevice, although it makes it possible to obtain a more linear variationin the gain, nevertheless has stability problems due to the feedbackloop formed by the MOS transistor.

Furthermore, as in the case of FIG. 2a, the voltage for controlling thegain comes from a digital/analogue converter 39 which occupies a largeamount of space, in particular when the assembly is produced inintegrated circuit form.

In the third case in FIG. 2c, a network of resistors 56 is placed in thefeedback path of an amplifier 57 which receives the input signalV_(IN3). The resistors 56 may or may not be connected in parallel usingswitching devices 59 which are controlled directly by a digital signalV_(COM3) output by the digital processing circuit 35. The gain isadjusted by connecting a greater or lesser number of resistors in thefeedback path. This device makes it possible, advantageously incomparison with the former two, to do away with a digital/analogueconverter 39. However, it also has stability problems due to theexistence of the feedback loop.

Because of the various problems explained above, the controlled-gainamplifiers of the prior art cannot be used satisfactorily in the systemsfor processing the signals S0 to S3 in FIG. 1 in order to adjust theamplitude of the signals delivered to the mulitplexer 15 and thereforeto the analogue/digital converter 18.

SUMMARY OF THE INVENTION

The object of the invention is to solve the various problems which havebeen explained above.

To that end, it provides a variable-gain amplifier device controlled bya digital signal, which has:

an input stage for converting the input signal into an input current;

a first series of reference current sources which are arranged in such away that the sum of the currents flowing through each of the saidreference sources is equal to the input current;

a second series of output current sources, in each of which the currentflowing through the first reference sources is duplicated and which arearranged in such a way that the sum of the currents flowing through theoutput sources is equal to the output current;

an output stage for converting the output current into an output signal.

According to the invention, the number of sources which are connected inthe first and second series of current sources is controlled by thedigital signal and determines the gain of the device.

The gain of the amplifier device is thus controlled directly by adigital signal and, since the device does not have a feedback loop, itis perfectly stable.

The invention also relates to an apparatus for reading optical discs,having a laser diode for emitting a light beam and photodiodes designedto pick up the light beam reflected by an optical disc when it is placedin the apparatus, the signals output by the photodiodes being processedin processing systems before being sent to an analogue/digital converterwhose output is connected to a digital circuit. According to theinvention, the processing systems of the apparatus have a variable-gainamplifier device controlled by a digital signal output by the circuitsuch as that described above for matching the signals sent to the inputof the analogue/digital converter to the dynamic range of the converter.

According to another aspect of the invention, the current flowingthrough the laser diode of the apparatus is roughly constant. Its lifeis thus extended by this.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will becomeapparent from the description below of several embodiments of theinvention, which is given with reference to the appended drawings, inwhich:

FIG. 1 already described above, schematically illustrates a part of anoptical disc reader apparatus according to the prior art;

FIGS. 2a to 2 c also described above, represent three analogue/digitalprocessing systems of the prior art having a gain adjustment stage;

FIG. 3 represents an amplifier device with digitally controllable gainaccording to a first embodiment of the invention;

FIG. 4 represents an amplifier device with digitally controllable gainaccording to a second embodiment of the invention;

FIG. 5 illustrates a part of an optical disc reader apparatusimplementing the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 represents a current/current amplifier which receives as input acurrent I_(IN) and delivers as output a current I_(OUT) as a function ofa digital control signal 200. The input current I_(IN) is applied to acurrent mirror formed by the N-channel MOS transistors 201 and 202. Moreprecisely, the current I_(IN) is applied to the drain of the transistor201 which is connected to its gate, itself connected to the gate of thetransistor 202. The drain-source current of the transistor 202 istherefore equal to I_(IN.)

A first series of M current sources which consist of P-channel MOStransistors T₇₁, T₇₂, T_(7M) is arranged in such a way that the sum ofthe currents of these sources is equal to I_(IN). All the gates of thetransistors T₇₁ to T_(7M) are connected respectively to the drains ofthe same transistors, which are themselves connected to the drain of thetransistor 202. All the sources of the transistors T₇₁ to T_(7M) areconnected to a supply voltage V_(CC) through a series of switches 171,172, 17M. The switches 171 to 17M are controlled by respective binaryinstructions a1, a2, aM constructed on the basis of the digital controlsignal 200 by a control logic circuit 210.

The drain-source current of the transistor 202, which is equal toI_(IN), is thus divided between the M current sources. If all theswitches 171 to 17M are closed, the source-drain current flowing througheach of the transistors T₇₁ to T_(7M) is equal to I_(IN)/M. Conversely,if only two switches out of the M are closed, then the current flowingthrough the corresponding transistors is equal to I_(IN)/2.

A second series of N current sources which consist of P-channel MOStransistors T₈₁, T₈₂, T_(8N) is arranged to form a current mirror withthe first series of M sources. That is to say all the gates of thetransistors Tel to T_(8N) are connected to the common point of the gatesand drains of the first transistor T₇₁ to T_(7M). The sources of thetransistors T₈₁ to T_(8N) are connected to the supply voltage V_(cc) andthe drains of the transistors are each connected to a first terminal ofa respective switch 181, 182, 18N, the second terminals of all theswitches 181 to 18N being connected together to form a node to which awire 205 delivering the output current of the amplifier I_(OUT) isconnected. The switches 181 to 18N are controlled by the respectivebinary instructions b1, b2, bN which are constructed on the basis of thedigital control signal 200 by the control logic circuit 210.

The current flowing through the sources of the first series (T_(7l) toT_(7M)), which will be referred to below as reference sources, isduplicated in each of the sources of the second series, which will bereferred to below as output sources. If m switches controlling thereference sources are closed, then the current flowing through each ofthe reference sources is equal to I_(IN)/m, and this current isduplicated in the output sources. If it is now assumed that n switchescontrolling the output sources are closed, the current delivered at theoutput of the amplifier is given by the following equation (1):$\begin{matrix}{I_{OUT} = {\frac{n}{m} \times I_{IN}}} & (1)\end{matrix}$

By modifying the numbers m and n appropriately, a variation in the gainof the amplifier is obtained. In comparison with the prior artamplifiers (in particular those represented in FIGS. 2b and 2 c), theamplifier of the invention has no stability problem because it does nothave any feedback loop.

It should be remembered that the purpose of the controlled-gainamplifier of the invention is to output a signal with quasi-constantamplitude corresponding to an optimum value allowing best use to be madeof the dynamic range of an analogue/digital converter arranged at theoutput of the amplifier, whatever the amplitude of the input signal ofthe amplifier is.

In the example in FIG. 3, it is therefore necessary for the outputcurrent I_(OUT) to be kept quasi-constant, whatever the value of I_(IN)is. If I_(IN) is small, the gain of the amplifier will have to be high,and n will need to be higher than m (referring to equation (1)).However, if I_(IN) is large, the gain of the amplifier will have to besmall and n will be chosen smaller than m. The number of closed switchesout of the switches 171 to 17M controlling the reference sources willtherefore vary inversely in relation to the number of closed switchesout of the switches 181 to 18N controlling the output sources.

It is furthermore desirable to have a quasi-constant increment betweentwo different values of the output current I_(OUT) which are obtainedfor two consecutive codes of the digital control signal 200 when I_(OUT)is closed to its optimum value. However, the minimum variation incrementof I_(OUT) corresponds to the current flowing through one of the outputsources T₈₁ to T_(8N), that is to say I_(IN)/m, assuming that m switchescontrolling the reference sources are closed. When the input currentI_(IN) varies, it is therefore possible to modify m in order to keep aquasi-constant minimum variation increment of I_(OUT) It is alsoimportant to have a small variation increment, in particular when thesystem is close to the optimum setting of the gain.

It is an advantage of the present invention that it makes it possible tocontrol the minimum variation increment of the output signal because, inthe amplifier devices of the prior art, this increment very oftendepends on the manufacturing process and is not controllable. Inparticular, in the example in FIG. 2b, the increment depends on thevariation in the value of the resistance formed by the MOS transistor 46placed in the feedback loop, the accuracy of which is not alwayssatisfactory.

A description of various control modes for the switches controlling thereference sources and the output sources will now be given.

In a first control mode, it is assumed that the digital control signal200 is a signal encoded using N_(c) bits. The number n of closedswitches out of the control switches of the output sources varieslinearly from 1 to N_(c). The number m of closed switches controllingthe reference sources varies in reverse to n from N_(c) to 1 accordingto the following equation: m=N_(c)−n+1. The output current of theamplifier is then: $\begin{matrix}{I_{OUT} = {\frac{n}{N_{c} - n + 1}I_{IN}}} & (2)\end{matrix}$

For a given application of the amplifier, it is desirable to have acurrent I_(OUT)=I₀ as output. The value of the number no which makes itpossible to obtain this value as a function of the current I_(IN)received at the input is given by equation (3) below: $\begin{matrix}{n_{0} = \frac{( {N_{c} + 1} )I_{0}}{I_{IN} + I_{0}}} & (3)\end{matrix}$

Knowing that the number no has to be a natural integer, the integer partof the number given by equation (3) will be taken. The number of bitsN_(c) of the instruction will therefore be adapted as a function of thedynamic range of the input current I_(IN), according to the precisionwhich it is desired to obtain.

The variation in the output current as a function of the instructioncode n is given by equation (4) below: $\begin{matrix}{\frac{I_{OUT}}{n} = \frac{I_{IN}( {N_{c} + 1} )}{( {N_{c} - n + 1} )^{2}}} & (4)\end{matrix}$

For a desired value of output current I_(OUT) =I₀, the variationincrement of the output current as a function of the input current,calculated by replacing n in equation (4) with the value no given byequation (3), is therefore: $\begin{matrix}{\frac{I_{0}}{n} = \frac{( {I_{IN} + I_{0}} )^{2}}{I_{IN}( {N_{c} + 1} )}} & (5)\end{matrix}$

According to calculations made by the inventor, by choosing N_(c)=6 bitsand I₀ equal to 1, when the signal I_(IN) varies from −20 dB to +20 dBrelative to I₀ (that is to say from 0.01 to 100), the variationincrement of the output current remains limited to 2 dB maximum.

In a second control mode, a digital control signal 200 encoded using 6bits is used, that is to say one which can assume 64 different values.The number of reference sources is 64 (M=64) and the switches 171 to 17Mare controlled thermometrically, that is to say by reducing the numberof closed switches by 1 when the received code increases by 1. When thecode “0” is received, all the reference sources are connected (all theswitches 17 x are closed) and when the code 63 is received, only onereference source is connected (only switch 171 is closed).

Conversely, the number of output sources is 6 (N=6), each beingcontrolled by one of the bits of the control signal 200, and sources areused whose size is weighted as a function of the significance of thecontrol bit. That is to say, for producing the output sources, MOStransistors T₈₁ to T_(8N) are used whose channel width W/L (W and Lbeing values characteristic of the width and the length of a MOStransistor gate) varies as a function of the significance of the controlbit. Since the current passing through a MOS transistor is proportionalto the width of its channel, the current flowing through each sourceT_(8x) will therefore be equal to: 2^(x)I_(IN)/m, assuming that I_(IN)/mis the current through each of the reference sources. This can besummarized by Table 1 below:

TABLE 1 Source T₈₁ T₈₂ T₈₃ T₈₄ T₈₅ T₈₆ Current I_(IN)/m 2 I_(IN)/m 4I_(IN)/m 8 I_(IN)/m 16 I_(IN)/m 32 I_(IN)/m

Finally, the strategy for controlling the reference and output sourcesas a function of the code received in the signal 200 can be summarizedin Table 2 below, in which the binary instructions a1 to aM and b1 to bNassume the value of “1” when the switch which they control is to beclosed, and the value “0” when the switch is to be open.

TABLE 2 Output source Code Reference source instruction instructionI_(OUT) = received a64 a63 . . . a3 a2 a1 b6 . . . b3 b2 b1 f (I_(IN)) 01 1 . . . 1 1 1 0 . . . 0 0 0 1 0 1 . . . 1 1 1 0 . . . 0 0 1 I_(IN)/632 0 0 . . . 1 1 1 0 . . . 0 1 0 (I_(IN)/62) × 2 3 0 0 . . . 1 1 1 0 . .. 0 1 1 (I_(IN)/61) × 3 . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . 61 0 0 . . . 1 1 1 1 . . . 1 0 1(I_(IN)/3) × 61 62 0 0 . . . 0 1 1 1 . . . 1 1 0 (I_(IN)/2) × 62 63 0 0. . . 0 0 1 1 . . . 1 1 1 I_(IN) × 63

It is clear that the largest codes will be used when I_(IN) is thesmallest, and consequently needs to be amplified, while the smallestcodes will be used when I_(IN) is large, the code “0” not being used inpractice.

Other control modes may also be envisaged. In particular, the number ofreference sources may be reduced in order to minimize the area of thecircuit and simplify the control. Provision may also be made for acertain number of reference sources to remain permanently connected.What is required in practice is to find a compromise between thespecifications expected of the circuit (expected output current I_(OUT),maximum increment allowed in variation of the output current, etc.) andthe size of the circuit as a function of the dynamic range of the inputcurrent.

The amplifier which has just been described in conjunction with FIG. 3is a current/current amplifier, but the principle of the invention isentirely applicable to a voltage/voltage amplifier. FIG. 4 representssuch an amplifier, which receives an input voltage V_(IN) and whichdelivers at its output a voltage V_(OUT) which is a function of adigital control signal 200. All the elements which are identical tothose in FIG. 3 have identical references, and will not be describedfurther. Only the input and output stages differ from those in FIG. 3.

The voltage V_(IN) is applied to the non-inverting input of anoperational amplifier 203. The output of the amplifier is connected tothe gate of an N-channel MOS transistor 204, while its inverting inputis connected to the source of the transistor 204 and to the firstterminal of a resistor 206, the second terminal of which is earthed. Thevoltage V_(IN) is therefore also found across the terminals of theresistor 206 through which a current I′_(IN) proportional to V_(IN)flows, this current I′_(IN) also being equal to the drain-source currentof the transistor 204. This transistor 204 fulfils the same function asthe transistor 202 of the amplifier represented in FIG. 3.

At the output, the current I_(OUT) passes through a resistor 207, oneterminal of which is earthed in order, at the other terminal of theresistor, to deliver the output voltage V_(OUT). In other regards, theway in which the amplifier in FIG. 4 operates is identical to that inFIG. 3.

FIG. 5 represents a part of an optical disc reader device whichadvantageously incorporates the amplifier which was described in FIG. 3.Elements which are similar to those in FIG. 1 which has already beendescribed have the same references and will not be described again.

The read head 5 delivers four signals S′0, S′1, S′2 and S′3 whichcorrespond to the currents output by the photodiodes A to F. Thesesignals are processed in four identical processing systems which lead tothe four inputs E′0, E′1, E′2 and E′3 of a multiplexer 115.

The processing systems each have a fixed-gain current/current amplifier110 to 113, followed by a controlled-gain amplifier 30 to 33 such as theone which is described in FIG. 3, a circuit 40 to 43 controlling theoffset of the signal which is arranged at the output of thecontrolled-gain amplifier, and a resistor 50 to 53 which makes itpossible to convert the current into voltage in order to apply it to anoperational amplifier 60 to 63, set up in follower mode, whose output isconnected to the input E′0 to E′3 of the multiplexer 115. The output ofthe multiplexer 115, which is a voltage signal, is sent to the input ofan analogue/digital converter 118 which delivers a digital signalencoded using 6 bits to a digital circuit 125 for processing anddecoding the signals output by the photodiodes.

The operational amplifiers 60 to 63 which are arranged at the end of thesystems for processing the signals S′0 to S′3 fulfil the function ofbuffers to prevent mixing, or crosstalk, between the channels of themultiplexer 115. It will be noted that, in contrast to the prior artdevice represented in FIG. 1, the current/voltage conversion of thesignals S′0 to S′3 is carried out before the signals are multiplexed.This is because, since the variable-gain amplifiers 30-33 make itpossible to modify the amplitude of the signals directly in theprocessing system, it is no longer necessary to provide an externalresistor whose value is selected according to the type of read head.There are several advantages with this: on the one hand, it limits thenumber of resistor references needed for manufacturing the device, andon the other hand it restricts the noise in the signal in comparisonwith the solution in which the resistor is external, this noise beingcreated by the difference in earth potentials between the internal earthof the circuit and the external earth.

The controlled-gain amplifiers 30-33 make it possible both to correctthe variations in amplitude of the signals S′0-S′3 which are due to thedifferent types of read heads used, and those which are due to thesurface condition of the optical disc 1 or other factors. Since theamplifiers 30-33 are controlled directly by a digital control signal108, constructed by the circuit 125 as a function of the signal receivedfrom the analogue/digital converter 118, it is no longer necessary toprovide an extra digital/analogue converter, and it is furthermore nolonger necessary to vary the current through the laser diode 2. Thecircuit is therefore more compact and the laser diode 2, whose currentis kept constant, has a longer life than in the prior art.

The invention is not of course limited to the embodiments which havebeen described above, and encompasses all variants. In particular, theamplifier with digitally controlled gain as described in FIG. 3 can beused in applications other than the circuit for processing the signalsoutput by a read head of an optical disc reader. In particular, it canbe used in the processing of video signals or audio signals output bytuners.

What is claimed is:
 1. A variable-gain amplifier device receiving aninput signal and being controlled by a digital signal, comprises: aninput stage for converting the input signal into an input current; afirst series of reference current sources which are arranged in such away that the sum of the currents flowing through each of said referencesources is equal to the input current; a second series of output currentsources, in each of which the current flowing through the firstreference sources is duplicated and which are arranged in such a waythat the sum of the currents flowing through the output sources is equalto the output current; an output stage for converting the output currentinto an output signal; wherein the number of sources which are connectedin the first and second series of current sources is controlled by saiddigital signal and determines the gain of said device.
 2. The deviceaccording to claim 1, further comprising first switches which arearranged in series with the reference current sources, and secondswitches which are arranged in series with the output current sources,the opening and closing of said switches being controlled by binaryinstructions constructed by a control logic circuit on the basis of saiddigital signal.
 3. The device according to claim 2, wherein saidreference current sources comprise P-channel MOS transistors, all ofwhose gates and all of whose drains are connected together and whosesources are connected to said first control switches, the output currentsources comprising P-channel MOS transistors, all of whose gates areconnected together and to the common point of the gates and drains ofthe reference sources and whose sources are connected to said secondcontrol switches.
 4. The device according to claim 2, wherein the number(m) of closed switches out of those controlling the reference currentsources is inversely proportional to the number (n) of closed switchesout of those controlling the output sources.
 5. The device according toclaim 4, wherein the digital control signal being encoded using N_(c)bits, the number n of closed switches out of those controlling theoutput current sources varies from 1 to N_(c), while the number m ofclosed switches out of those controlling the reference current sourcesvaries from N_(c) to 1 according to the equation m=N_(c)−n+1.
 6. Thedevice according to claim 2, wherein the output current sources are eachcontrolled by one bit of said digital control signal, the size of thesources being weighted as a function of the significance of the bit ofsaid control signal.
 7. The device according to claim 6, wherein thedigital control signal is encoded using 6 bits, the first series ofreference current sources comprising 64 sources of the same size, andwherein the second series of output sources comprises 6 sources of sizesweighted by the significance of the bit of the control signal.
 8. Anapparatus for reading optical discs, comprising a laser diode foremitting a light beam and photodiodes designed to pick up the light beamreflected by an optical disc when it is placed in the apparatus, thesignals output by the photodiodes being processed in processing systemsbefore being sent to an analogue/digital converter whose output isconnected to a digital circuit, wherein said processing systems have avariable-gain amplifier device controlled by a digital signal output bythe circuit according to claim 1 for matching the signals sent to theinput of the analogue/digital converter to the dynamic range of saidconverter.
 9. The apparatus according to claim 8, wherein the currentflowing through the laser diode (2) is roughly constant.